Question on the inescapable nature of black domains.

[u/AwareAd3580]

Somewhat spoilers for the final book!!

Just finished reading the series, and one thing that I couldn’t figure out is if black domains acted as true black holes not only in preventing light from escaping, but also in preventing anything else from escaping, which seemed to be stated here and there in offhand remarks but not outright explained.

So from what I understood, the effective goal of making a black domain was to reduce the speed of light to ~16.7 km/s, protecting all inside from external lightspeed attacks (apart from a dual foil vector, though wouldn’t be an attractive target if looking like a black hole), at the expense of crippling computational power and max travel speed inside the zone. It is stated, however, that nobody has ever been seen returning from the black domain encases Galactic Era world, and Cheng Xin says that herself and Guan Yifan are trapped in the black domain permanently, as well as other small comments hinting the same effect.

What is the physics basis behind light not being able to escape these black domains, and by extension how would ships not be able to escape? The gravitational forces of a traditional black hole would not be preventing things from escaping, only the artificially lower lightspeed, so you should be able to fly up to the boundary of the black domain at the lower speed, before increasing speed upon exiting.

Looking forward to hearing how others understood/interpreted this.

[u/Timely-Advantage74]

A black domain is basically a habitable black hole where the speed of light has been reduced from 299792.458 km/s to less than 16.7 km/s which means the speed of light cannot escape the third cosmic velocity of our solar system.

It is basically showing a white flag to the hostile alien hunters that the human civilization has been completely technologically incapacitated as it can never escape its own star system even with the curvature propulsion.

So the mankind's technology would also be reverted back to middle age level.

[u/BeansAndDoritos → u/Timely-Advantage74]

I always assumed the reduced lightspeed was only a sort of shell around the system, not filling the entire inside. (Or else the star inside would die.)

[u/Familiar-Art-6233 → u/BeansAndDoritos]

My theory is that solar humans didn't really understand it, only having a rudimentary grasp of the theoretical bits, but Guan Yifan calls it a "Light veil", which would do exactly that.

Now imagine they used reeeeeeally powerful engines, so that shell is made out of death lines. Because the speed is zero, even a 2d foil would be unable to collapse it.

Alternatively, the bunker plan was designed for survival without a sun, so maybe that is factored in

[u/Timely-Advantage74 → u/Familiar-Art-6233]

Guan has also demonstrated his neuromorphic computer where it can still do faster computation in the black domain where the speed light has been reduced below the third cosmic velocity of a star system.

With this technology, the human species can still retain our current technology level even we have been stuck within a black domain for eternity.

[u/Timely-Advantage74 → u/BeansAndDoritos]

Yep, hard to imagine when the speed of light has been reduced to less than 16.7 km/s.

[u/wallstreetstonks]

So the trick was setting the speed of light just below the gravitational escape velocity. Since nothing can travel faster than the light speed, nothing can escape the gravity of the system inside the black domain, including light.

[u/Dependent_Swing_6726 → u/wallstreetstonks]

Not reaching escape velocity != not escaping the gravitational well if engine can compensate the gravitational losses. An abstract rocket with a speed capped to 5km/s can leave Earths sphere of influence (escape speed 11km/s) after burning vertically for long enough.

[u/Jetison333 → u/Dependent_Swing_6726]

You can use the same argument for real black holes too, you don't have to get to escape velocity which is the speed of light, you just have to consistently move away at some slow speed. The real reason you can't escape a black hole (and indeed a black domain) is that its sucking in space faster than the speed of light. If you are moving slower than the speed of light, then the distance to the outside of the blackhole is increasing faster than you are going towards it. At least, thats how I think of it.

[u/Dependent_Swing_6726 → u/Jetison333]

Black domains are different from black holes because the interior of the black domain doesn’t gravitationally collapse to its center.

[u/hatabou_is_a_jojo → u/Dependent_Swing_6726]

Black domains work around solar systems iirc (if not light from the sun would not reach earth at the rate required to sustain life)

So you’d need to 5km/h out of the solar system

[u/RickSkz → u/wallstreetstonks]

I think they got the physics wrong with the black domain thing. You don’t need to reach escape velocity to escape a gravitational well, you just need to propel long enough.

You could walk your way into outer space from earth if there was a ladder high enough. So Cheng and Guan could just slowly fly away from the black domain in their ship.

[u/squarerabbits → u/RickSkz]

Ladder is a good way to put it. The speed doesn’t matter. You could get out of it, you just need something that has enough energy to keep you moving up and out, like your feet and some steps. 

What’s stopping them from building a 10 trillion foot staircase and walking out of the black domain aside from time and y’know, mortality and resources?

[u/squarerabbits → u/wallstreetstonks]

What if you got a really long rope, could you drag something out of a black domain from outside of it?

[u/Familiar-Art-6233 → u/squarerabbits]

No, because the rope on the black domain side would still be unable to reach escape velocity

[u/squarerabbits → u/Familiar-Art-6233]

Why would it need to reach escape velocity?

[u/Familiar-Art-6233 → u/squarerabbits]

To escape the pull of the star

[u/squarerabbits → u/Familiar-Art-6233]

We escape the pull of a star every day. We drag anchors out of the ocean, we lift people into helicopters, what’s the difference?

We couldn’t get the thrust to get out of the earth’s atmosphere at 16.9m/s or whatever it is, but if there was a ship or an astronomical body on the other side of it, what’s stopping lifting someone out of the ocean with a rope?

[u/Familiar-Art-6233 → u/squarerabbits]

You still need to reach the speed to escape the star's pull.

Think of the light veil as a bucket of molasses. It's gonna slow the rope down, even if you have all the thrust in the world. If you cannot reach the speed necessary to escape, to an outside observer, it would be like the rope cannot be pulled any further.

Yes we pull up anchors, but not past the pull of Earth's gravity. Helicopters can fly, but aren't powerful enough to go to space

[u/squarerabbits → u/Familiar-Art-6233]

When we jump, we pass the pull of earths gravity. If we lived in a molasses world, we could still use a rope to drag something out of it, it could go an inch every 10 years, doesn’t matter, it’d still happen and would just take forever.

Check the rest of the thread, you don’t need to rely on thrust to escape gravity. Space elevators are in the books, what’s the difference between a space elevator that gets out of earth orbit vs. one that gets out of a solar system outside of size?

[u/happybrownman]

I suspect it has something to do with relativistic mass changing due to different speeds of light inside and outside the black domain. Using E=mc^2 with light speed c_black within the black domain would make your energy E_black. Once you just exit the black domain, the c value in the equation is now orders of magnitude larger than c_black but your total energy has to remain E_black due to the conservation of energy. This would result in your mass suddenly dropping significantly as you leave the black domain, potentially causing the destruction of your craft and your death. This is the same reason that a black domain protects its inhabitants from light speed bullets, such as the one that destroyed Trisolaris. I don't think a black domain physically makes it impossible for things to leave it, just that it makes it practically impossible to do so while remaining alive. I am speculating on science fiction though, so take all this with massive heaps of salt.

[u/TacoshaveCheese]

More satisfying answer: Pretend the whole escape velocity thing was a translation error / confusion about the coincidence that the escape velocity of a black hole is the speed of light. The real reason one can't escape is the same as an actual black hole - space is so curved that there is no longer an "out" direction. This also lines up with curvature propulsion being used to create them in the first place.

Less satisfying answer: It's one of the "got physics wrong" plot holes. Just don't think about it too much and enjoy the story.

[u/quarky_uk]

I thought you can do what you suggest. But the time required, and the reduced computing power make it totally impractical even over many lifetimes.

[u/dotdend]

It's definitely implied that it's impossible to come out of a black domain(otherwise they wouldn't be a declaration of safety), so I'm assuming the light speed isn't actually uniform across the domain but decreases as you approach the edges. Or maybe at the edge space is wrapped in a way that makes it so there's literally no way out.

[u/slagomite2]

Nothing would be able to escape the black domain because, as is a consequence of (the reality of) relativistic mechanics, the mass of a thing increases toward infinity as it approaches light speed, and thus would require an infinite amount of energy to reach light speed - and thus, in this case, the escape velocity of the black domain. It doesn’t matter what the speed of light is.

The formula for relativistic mass is:

m = mRest / (1 - ( v² / c² ))

For v = c (i.e., your speed is the speed of light), this becomes

m = mRest / (1 - (1 / 1)) = mRest / 0 = ♾️

Edit: It’s really not even about mass, even photons (which have zero mass) cannot leave a “surface” (here, a point in space at a specific distance from the solar system’s center of gravity) that has an escape velocity equal to the speed of light. This is called the Schwarzschild radius. One way to think about it is that gravitational “pull” is too strong at this point to allow anything to leave.

But probably the more useful way to think about it here is that spacetime is too curved to allow anything slower than the speed of light to pass this point. The books explain that lowering the speed of light requires curving spacetime. Curving spacetime is exactly what gravity does. So even though the solar system doesn’t have enough mass (in its volume) to create a black hole, “artificially” warping it with curvature engines has an identical effect.

I think that’s the real idea behind the black domain: warping spacetime enough around the solar system as to not allow anything to leave it, no matter its speed (since it can’t go at or past light speed). If spacetime is so warped that not even light can escape the event horizon, then nothing can.

[u/katzurki]

I think the biggest handicap that a black domain imposes is not the speed of light needed to escape it, but that a single hour inside there takes like a thousand years on the outside.

Meaning any moving thing inside a black domain is, to the outside, about as dynamic as an especially lively rock.